The most common and most likely answer to this crossword clue is the 5 letter word BLOOM
I've seen this crossword clue in The Times
B
L
O
O
M
More clues leading to the result BLOOM
- Left in period of prosperity to flourish
- Open book has become visible
- Mature sweeper changes sides
- Bishop to emerge and thrive
- Bachelor can go to McDonalds, it’s opened
- Flower‘s sudden growth around lake
- Flourish, due to return of low pound
- Flower beginning to limp in thunder
- Old car firm on outskirts of Orlando on motorway to Orlando
- Success securing student ideal of perfection
Buddy explains!

The straight part
We know that "Open" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case open = bloom.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BLOOM mean?
BLOOM noun- A blossom; the flower of a plant; an expanded bud.
- Flowers, collectively.
- The opening of flowers in general; the state of blossoming or of having the flowers open.
- A state or time of beauty, freshness, and vigor; an opening to higher perfection, analogous to that of buds into blossoms.
- Rosy colour; the flush or glow on a person's cheek.
- The delicate, powdery coating upon certain growing or newly-gathered fruits or leaves, as on grapes, plums, etc.
- Anything giving an appearance of attractive freshness.
- The clouded appearance which varnish sometimes takes upon the surface of a picture.
- A yellowish deposit or powdery coating which appears on well-tanned leather.
- A bright-hued variety of some minerals.
- A white area of cocoa butter that forms on the surface of chocolate when warmed and cooled.
- An undesirable halo effect that may occur when a very bright region is displayed next to a very dark region of the screen.
- To cause to blossom; to make flourish.
- To bestow a bloom upon; to make blooming or radiant.
- Of a plant, to produce blooms; to open its blooms.
- Of a person, business, etc, to flourish; to be in a state of healthful, growing youth and vigour; to show beauty and freshness.
- (Cooking) To bring out the flavor of a spice by cooking it in oil.
- The spongy mass of metal formed in a furnace by the smelting process.